1. Technical Field
The present invention relates to texturizing and polishing disks. Specifically, the present invention relates to a method and apparatus for texturizing disks of the type used for magnetic storage of information.
2. Discussion of the Related Art
During operation, magnetic storage disks spin at high speeds while a read/write head floats on a cushion of air near the surface of the disk. When not in use, the read/write head typically "parks" in a special zone of the disk, usually near the inside edge. The disk surface is texturized in order to prevent the head from sticking onto the surface of the disk when parked. The texture on the disk consists of many small grooves, typically on the order of 40 angstroms from peak to valley, and covers the entire disk even though the disk head parks in only a small zone of the disk.
Texturization of magnetic storage disks is generally accomplished by rotating the disk while bringing to bear upon each side of the disk an abrasive medium supported by a compliant roller. The position of the roller and medium relative to the disk is made to vary toward and away from the center of the disk in an oscillatory manner. The resulting relative motion between the disk surfaces and the abrasive media has two principal components, one due to the rotation of the disk and another due to the oscillatory motion of the abrasive media.
Although the abrasive medium is generally several centimeters wide and contains many thousands of abrasive particles, it is useful to discuss the path traced by a single particle on the surface of the disk. The path of an abrasive particle on the disk will have a continuously varying radius of rotation about the disk center. A texture groove formed by such an abrasive particle thus creates a non-circular pattern having a variable radius. This pattern depends on the oscillatory motion between the disk and medium as well as the rotation of the disk.
It is generally undesirable for the path of a particle to repeat, meaning that the particle retraces the same path on successive rotations of the disk. Repeating patterns cause excessive concentration or depth of texture grooves on the surface of the disk. Repeating patterns are generally caused by unintentional synchronization between disk rotation and oscillation frequency.
It is also undesirable for the texturized disk to have angles between the grooves and path of the read/write head (the "crossing angle") smaller than a given amount. For most applications, crossing angles of the texture grooves should be more than 20 degrees. If the crossing angles of the texture grooves are too small, the read/write head may position incorrectly during operation.
Texture grooves having undesirably small crossing angles are formed when the oscillation velocity (the relative velocity between an abrasive particle and the center of the disk) is too low compared to the rotational speed of the disk. In order to minimize the formation of grooves with low crossing angles, the magnitude of the oscillation velocity ideally should be held above some level. Therefore, sinusoidal oscillating patterns are generally less desirable, since there is a relatively long period of time when the oscillating velocity is low. Conversely, triangular oscillating patterns are idea, since the oscillation velocity is held at a constant magnitude, varying only in sign.
One method of continuously varying the radius of a particle path during the texturizing process involves holding the spindle, about which the disk rotates, stationary with respect to translation while causing the abrasive media rollers and magazines to oscillate translationally in the plane of the disk and along a line coincident with the center of rotation of the disk. An example of an oscillating magazine machine is the model 1800 Automated Surface Finisher made by Exclusive Design Company, which is the assignee of the present invention.
Several disadvantages of oscillating magazine systems stem from the fact that these systems can only oscillate at relatively low frequencies. In order to oscillate both front and back tape rollers and magazines, a structure weighing approximately 95 pounds is continuously accelerated and decelerated. With such a massive structure, the maximum frequency is approximately 10 Hertz using relatively small sinusoidal amplitudes such as 0.01 inches. At amplitudes typical to most applications, such as 0.125 inches, the maximum frequency is approximately 4-5 Hertz. In order to achieve acceptable crossing angles, operating at such low frequencies requires an extremely slow rotational speed for the disk. Thus, overall throughput of such systems is unacceptably low.
Another disadvantage of systems restricted to low oscillation frequencies is that it is difficult to avoid the natural resonance frequencies of the machine. In systems capable of higher oscillating frequencies, the frequency may be increased in order to avoid unacceptable vibrations caused by resonance.
Further problems arise with oscillating magazine systems when designing new texturizing patterns. Traditionally fixed amplitude sinusoidal oscillation patterns have been used. However, disk storage manufacturers increasingly require texture patterns with a semi-triangular oscillation waveform (triangular with parabolic turnarounds). In order to alter the shape of the oscillation curve, each bit of the waveform data file must be individually set by a programmer. In such a system, it is extremely difficult for the user to visualize and to then program an oscillation shape other than a sinusoid. The added programming expense and the likelihood of a long trial-and-error period normally precludes the user from changing the oscillation pattern.
Another problem related to the system's difficulty in programing is that it is difficult for the user to avoid particle path repetition when choosing an oscillation frequency and a disk rotation speed. In order to avoid retracing the same path on successive rotations of the disk, the user is forced to extensively experiment with various speeds and frequencies.
A second method of providing oscillatory motion between the rotating disk and the abrasive media is to hold the abrasive media and rollers stationary while causing the rotating disk to oscillate in directions parallel to the roller contact line. This oscillatory motion is generally accomplished by means of a cam and follower arrangement, wherein the cam is driven by a constant-speed servomotor. A schematic illustration of a cam system is shown in FIG. 1. As shown, disk 100 is held and rotated by spindle 102 which is driven by servomotor 104. Spindle 102 and servomotor 104 are both mounted on base 130. Cam followers 124 and 126 are held to the surface of cam 122 by force exerted by a preload spring 128. As cam 122 rotates, base 130 oscillates by sliding on linear bearings 132. Disk 100 is thus oscillated relative to a fixed abrasive media roller (not shown). An example of such a spindle oscillation method is illustrated by the model 1800A Automated Surface Finisher made by Exclusive Design Company, the assignee of the present application.
In a cam system, the oscillating spindle and disk are relatively light compared to the abrasive rollers and magazines. Thus, by operating at higher frequencies, the cam system alleviates many of the problems which plague oscillating magazine systems.
However, cam systems have a number of disadvantages. For example, with a cam system the user can easily change the frequency of oscillation and the rotation speed of the disk, but cannot easily change the amplitude or the shape of the oscillation pattern. Adjusting the amplitude or shape of the oscillation pattern requires designing, tooling, and installing an entirely new cam.
The disadvantage of not being able to easily change the amplitude and oscillation shape is especially troublesome when trying to design texturizing patterns having a high degree of triangularity. Although the ideal pattern for avoiding low crossing angles is a perfect triangle, in practice it is necessary to compromise between a triangle and sinusoid since a true triangle pattern requires an infinitely large acceleration of the spindle mass. Thus it is necessary to compromise by using a semi-triangular waveform which limits the maximum acceleration to acceptable levels. This semi-triangular waveform consists of straight lines joined by parabolic sections where direction reversal occurs. The goal is to provide the most triangular waveform possible without exceeding the acceleration limits of the system. For a given frequency and amplitude of the overall waveform, the system's maximum acceleration determines what proportion of the waveform is required to be parabolic. Associated with each frequency and amplitude is an optimum waveform. Thus, not only are different cams necessary for different displacements, but the optimal semi-triangular cam shape is different for every frequency at a given displacement.
Moreover, the problems in designing waveforms are compounded by the fact that the user cannot easily visualize or model a given cam's displacement waveform or the resultant texturizing pattern from a given cam. The texturizing pattern resulting from a given cam is a product of cam shape, rotational speed of both the cam and the spindle, as well as the particular synchronization between rotation and oscillation. Since there is no easy way to visualize or model the resultant texturizing pattern, an extensive amount of experimentation is required when using cam systems.
Problems with maximum acceleration and oscillation frequency arise when using a cam system. For example, the maximum acceleration of the cam system is limited by the preload spring, since at some point the cam followers can no longer be held to the cam surface. On the other hand, if the spring load is increased, excessive wear may occur in the cam followers and the cam surface.
Small variations in cam size and shape as well as variations in the speed of the spindle rotation may result in a substantial degree of variation in the quality of texturization of the disk surface. Variations in cam size and shape may be due to wear, or may be inherent in their manufacture. In related art systems, the accuracy of the spindle rotational speed cannot be accurately controlled since a smooth drive belt must generally be used in order to avoid velocity ripple effects on the texturizing pattern. By using a smooth drive belt, the spindle may experience a significant amount of slip.
Thus, the yield in prior art cam systems is often undesirably low since slight variations in the cam shape or spindle rotation speed may cause a retracing of the same path on successive rotations or unacceptable crossing angles.
As mentioned, magnetic storage disks have different zones, such as a landing or continuous start/stop zone, and a data zone. For disk storage systems, there is an optimal texture intensity and crossing angle for different zones. Therefore, it is desirable to be able to individually specify the crossing angles for each zone. For a given amplitude and frequency, the tangent of the crossing angle at a given point is inversely proportional to the radial distance of that point. This is due to the fact that the outer portions of the disk are moving faster and therefore have smaller crossing angles for a given frequency and amplitude. None of the related art systems allow the user to easily vary the oscillation amplitude or frequency with the mean radius.
Therefore, related art systems cannot easily provide the type of tapered texture areas which are desirable in future magnetic disk storage systems. For example, although the disk head currently only parks in a small zone of the disk, typically near the inner edge, the entire disk is texturized. In general, full texturization is used because the transition between texturized and untexturized zones of the disk when Using the prior art systems is too abrupt. If the transition between textured and untextured zones is not sufficiently smooth, the head operation will be disrupted.